Minutes ago I was having an amazing day of nerdery over "Doctor Who" at Wizard World's Philadelphia Comic Con. Posing in front of the best custom built TARDIS I have ever seen, made by Michael C. Collectibles (made out of pine, perfect color and weathering, just a thing of beauty). Having left my own Eleventh Doctor sonic screwdriver at home, I was borrowing one from a cosplayer and doing my best Time Lord stance.

It is worth noting that my first Doctor was Tom Baker, but Smith has become my favorite. As a professional journalist, I've been fortunate to interact with him quite a bit over the last three years of his four-year run; he has always been a great interview and an exceptionally gracious dude every time we've spoken. As a fan, I've loved what he has done with the role, making the Doctor this old-man in a young man's body. His Doctor is mischievous, dark, playful, brooding.

So yeah, like many Whovians, I am bummed to see him leave the role. Even Doctor Who executive producer Caroline Skinner (who has also left the show) joked to me last September -- when asked about the prospects of Smith leaving the show -- that "we’re never going to lose Matt!"

But hey, time does pass and change is a big part of the Who-niverse. We all knew he wouldn't be the man from Gallifrey forever.

"The show is about change," he said. "Like Steven likes to say, it can never be predictable, it can never be cozy – It’s got to feel like it’s sort of marking new territory, I think, every season."

Still, his departure creates a tricky challenge for BBC America, especially considering Smith has become the first Doctor fully embraced by mainstream pop culture stateside. As much as I love David Tennant, it was Smith that made the cover of Entertainment Weekly, TV Guide and was featured in Rolling Stone.

For her part, Skinner told me that she thought American audiences could roll with the change, however.

"I think that everyone, including the U.S. fans, also seem to really get and love the fact that Doctor Who is kind of all about change and change is its essence, and it must never stop moving because that way the show won’t really be at its full strength."

So let the speculation begin, which will no doubt consume nerdy conversations over the next several months.

Will the next Doctor indeed be John Hurt? After Tennant and Smith became the objects of fan crushes, a return to an older Doctor would be unique and brave. And might we see a female Doctor or a person of color in the role?

We shall see. But as a final thought, I leave you with Smith's own comments to me (as part of a larger interview last December) about his time in the TARDIS and if he still felt lucky to be a part of the show:

"Oh man, every day playing that character, because it’s limitless. It’s boundless. I’m more so very fortunate that I have Steven Moffat at the helm of it, who, in my eyes, is a complete genius. So, I mean, honestly, not sounding too happy about myself, but it’s every day that I’m grateful for it because it offers you so much opportunity, especially as an actor ... [And] one of the most exciting parts of my tenure, really, is seeing the growing evolution of the show in the States - and just the general awareness of the show. I think the viewing figures - it was the biggest selling show on iTunes last year. TV Guide, Entertainment Weekly - things like that are real big steps for the show, and I feel very proud to be a part of it. It is nice to work on something that feels like it’s in a moment, or of a moment, you know?"